1. Field of the Invention
The present invention generally relates to a semiconductor manufacturing apparatus, and specifically to a plasma CVD film formation apparatus that comprises a shower plate and a susceptor having unique shapes and achieves favorable wafer temperature distribution as well as film thickness and quality distributions.
2. Description of the Related Art
FIG. 1 shows an overview of a conventional plasma processing apparatus. This plasma processing apparatus 1 has a reaction chamber 6, a gas introduction port 5, and a second electrode comprising a susceptor 3 and a heater 2. The second electrode may be structured with its susceptor 3 and heater 2 brazed together, as disclosed in U.S. Pat. No. 3,207,147. Gas is introduced from the gas introduction port 5 through a gas line (not shown). A round-shaped first electrode 9 is placed immediately below the gas introduction port 5. The first electrode 9 has a hollow structure, and its bottom surface has many pores through which gas is injected toward a wafer 4. To facilitate maintenance and reduce parts costs, the first electrode 9 is also constructed in such a way that a shower plate 11 having multiple gas introduction holes can be replaced.
At the bottom of the reaction chamber 6 is an exhaust port 10. This exhaust port 10 is connected to an external vacuum pump (not shown), through which the interior of the reaction chamber 6 is evacuated. The susceptor 3 is placed parallel to and facing the first electrode 9. The susceptor 3 holds the wafer 4 on top, and heats the wafer 4 continuously by means of the heater 2 to maintain the substrate 4 at a specified temperature (150 to 450° C.). The gas introduction port 5 and first electrode 9 are insulated from the reaction chamber 6, and connected to a first high-frequency power supply 7 provided externally. In some cases, a second high-frequency power supply 8 is also connected. Numeral 12 indicates grounding. Consequently, the first electrode 9 and second electrode function as high-frequency electrodes and create a plasma reaction field near the wafer 4. The type and quality of the film formed on the surface of the wafer 4 vary in accordance with the type and flow rate of material gas, temperature, radio frequency type, spatial distribution of plasma, and potential distribution.
To form C-doped silicon oxide film offering a low dielectric constant, silicon hydrocarbons containing multiple alkoxy groups is used in combination with Ar and/or He. In some cases, alcohols, ethers, unsaturated hydrocarbons and the like may be used as bridging gases. In other cases, oxidizing gases such as ozone, oxygen and nitrous suboxide may be used.
The uniformity of the film formed on the wafer is closely correlated to the gas holdup time and plasma density in the reaction area. As shown in FIG. 1, the distance between the susceptor 3 and shower plate 11, or distance between the wafer 4 and shower plate 11, remains constant on the conventional plasma CVD film formation apparatus. Generally on a parallel-plate plasma CVD film formation apparatus, the distribution of electric field strengths between the two plate electrodes (Ø250 mm) presents a trend where the electric field strength becomes the greatest at the center and decreases gradually towards the outside in the radial direction, as described in U.S. Pat. No. 6,631,692. Based on this distribution trend, the electric field strength is approx. ±5% in the film deposition area on a Ø200 mm wafer, and the distribution band increases further on a ◯300 mm wafer. For this reason, the electric field near the center of the wafer 4 is relatively stronger than in the electric field away from the center in the radial direction, and accordingly the plasma density and gas reaction rate are also higher near the center. In addition, although the gas reaction rate is affected by the holdup time of gas in the reaction space, in an example using a conventional plasma CVD film formation apparatus the film thickness became the greatest near the center, indicating that the film thickness distribution is strongly affected by the plasma density. In cases like this, the film thickness distribution has traditionally been corrected by controlling the gas flow rate, mixing ratio of component gases, pressure, radio frequency and radio frequency power output. However, changing these parameters also changes the film thickness and deposition speed, which in turn reduces the stability of the process. In addition, oftentimes the film quality does not have a uniform distribution unless the wafer temperature distribution is uniform.
FIG. 2 illustrates the susceptor 3 and shower plate 11 reported by the inventors of the present invention in a technical bulletin published by the Japan Institute of Invention and Innovation (Technical Disclosure No. 2002-1338). In this figure, the susceptor 3 is placed on top of a heater 2 and the susceptor surface on which a wafer 4 is supported has a concave shape. The distance (db) from the back face of the wafer 4 to the center of this concave surface (the depth of the concave) is 0.1 to 4 mm, and it is stated that the wafer 4 should ideally contact the susceptor 3 only at its periphery. The shower head 11 has a convex shape, and the deformation rate (fd) of the shower head 11, as defined by the equation (dc−da)/da×100 (wherein dc is the distance between the showerhead and wafer at the center, and da is the distance between the showerhead and wafer at the periphery) is in a range of −1% to −20%. The report states that this shape allows for formation of film having uniform thickness and quality. However, the inventors of the present invention subsequently discovered that the aforementioned structure would not provide a uniform wafer temperature distribution, nor would it provide uniform film thickness or quality distribution, if wafers of large areas, such as those with a diameter of 300 mm, are processed.